Inflatable fabrics used in several space, military, and consumer applications, including airbags, parachutes, rafts, boat sails, and inflatable shelters have a silicone coating over the fabric, which imparts special performance characteristics to the inflatable structure. For example, with respect to airbags for aerospace applications (such as those used for landing the Mars Pathfinder on the surface of Mars), silicone coatings improve the seam strength, resist unwanted permeation of air through the fabric, protect the base fabric from being degraded by the environment, and provide chemical and environmental stability to the fabric (Cadogan, D., Sandy, C., and Grahne M., “Development and Evaluation of the Mars Pathfinder Inflatable Airbag Landing System” 49th International Astronautical Congress, Sep. 28-Oct. 2, 1998/Melbourne, Australia). Additionally, silicone coatings exhibit one to two orders of magnitude less erosion compared to their organic counterparts when the airbag is in low earth orbit (LEO). Silicones (also called siloxane polymers) form SiO2 barrier films when exposed to atomic oxygen in a terrestrial environment (D. P. Dworak and M. D. Soucek, Progress in Organic Coatings, 47, 448 (2003), and Richard H. Parker, US Patent application, US 20040058601 (2004). Improving the mechanical and thermal properties of silicone coatings would enable the use of lighter fabrics, thereby leading to weight reduction of inflatable structures.
Specifically, silicone coatings with superior mechanical properties (e.g., tear strength, puncture resistance and tensile strength) will allow reductions in the mass and thickness of coated fabrics. For example, the mass limit on the airbag system that was used on the pathfinder for the Mars mission was 32 kg. By reducing the mass and volume of the airbags, engineers will have greater flexibility in designing highly compacted deflated structures that survive deployment, protect astronauts and the fabric from excessive heat, maintain structural performance, and effectively contain gases used for inflation and/or breathing atmospheres in space.
Previously, it has been shown that nanoparticles, specifically clay nanoparticles, can improve the mechanical properties (e.g., tensile strength) of cross-linked silicone matrices and poly(dimethylsiloxane) (PDMS) elastomers (F. X. Deneubourg, A. Beigbeder, Ph. Degée, P. Viville, and Ph. Dubois, http://morris.umh.ac.be/SMPC/Posters/2006-BPG-FRDE.pdf, Schmidt, D. F., “Polysiloxane/Layered Silicate Nanocomposites: Synthesis, Characterization, and Properties”, Ph.D. Dissertation, Cornell University, May (2003), and H. Takeuchi and C. Cohen, Macromolecules, 32, 6792 (1999)). However, until now no commercial efforts have been made to improve the performance of “commercial” silicone coating formulations using nanomaterial additives. Some commercial silicone coating formulations contain filler particles, e.g. liquid silicone rubber (LSR) from Dow Corning contained 15-40 wt % of modified silica particles. The present invention has demonstrated that a “dual nanocomposite system” can enhance the mechanical properties (e.g., tear strength) of a silicone coated fabric without significantly degrading other important coating properties, such as hardness, thermal stability and puncture resistance. The present invention provides significant benefits to the field of silicone coatings since the focus is not to change the chemistry of individual coating components, but rather to combine the attributes of two fundamentally different nanophase additives (also referred as nanomaterials).
Although not limited to one type of silicone coating, the focus of this application is on liquid silicone rubber (LSR) elastomers, which are used for a broad range of industrial fabric (e.g., Fiberglass®, Kevlar® {poly-paraphenylene terephthalamide}, Nylon, and Vectron® {liquid crystal aromatic polyester}) coating applications, including automobile airbags. Silicone elastomers on cross-linking form a three dimensional network. LSR is a two components coating formulation containing part A and part B and cured by an addition reaction as described below:
This reaction can be accelerated by heat, and it has no byproducts. The coatings are typically cured by heating at 160-180° C. for a few minutes. The “potlife” of the material varies with the inhibitor/catlayst level. Generally, most LSRs have a 24-hour potlife. LSR coatings have good chemical inertness, and they are highly resistant to water, weak acids and bases, polar solvents and a wide variety of chemicals.
Fillers, such as amorphous silica, are often added to the silicone matrix to give greater reinforcement [Rajan, G. S., Sur, G. S., Mark, J. E., Schaffer, D. W., and Beaucage, G., “Preparation and Characterization of Some Unusually Transparent Poly(dimethylsiloxane) Nanocomposites”, J. Poly. Sci., Part B: Poly. Phys., 41, 1897-1901 (2003)]. Calcium carbonate has been shown to be effective in improving combing strength for airbag applications when using a peroxide curing system [Dumont, L. and Pouchelon, A., “Silicone Composition and Process that is Useful for Improving the Tear Strength and Combing Strength of an Inflatable Bag for Protecting an Occupant of a Vehicle”, U.S. Patent Application US2005/0137321 A1, Jun. 23, 2005]. Layered silicates based on unmodified or organically modified montmorillonites have also been explored [Schmidt, D. F., “Polysiloxane/Layered Silicate Nanocomposites: Synthesis, Characterization, and Properties”, Ph.D. Dissertation, Cornell University, May (2003)]. These materials exhibited similar performance as those of traditional silicones reinforced with silica. In-situ techniques especially for forming silica or titania have also been used with some success for creating nanocomposites [Rajan, G. S., Sur, G. S., Mark, J. E., Schaffer, D. W., and Beaucage, G., “Preparation and Characterization of Some Unusually Transparent Poly(dimethylsiloxane) Nanocomposites”, J. Poly. Sci., Part B: Poly. Phys., 41, 1897-1901 (2003)]. Poly(dimethylsiloxane)—zirconia nanocomposites have been produced, but the focus of this work was to produce hybrid gels, which are annealed at a high temperature (>1000° C.) to produce nanocomposite ceramics or glasses [Dire et al., Chem. Mater., 10, 268 (1998)].
Zirconia, as well as other ceramic materials, are also used as fillers in polymer materials [Kolb, B. U. and Chien, B. T., “Zirconia Sol, Process of Making and Composite Material”, U.S. Pat. No. 6,376,590 B2 (2002)]. In trying to produce zirconia-based nanocomposites, several approaches have been used for forming the zirconia in-situ. These include zirconium precursors based on zirconium alkoxides, complexes with organic or inorganic molecules, and water-soluble salts. The formation of zirconia in polymethylmethacrylate (PMMA) has been studied to some extent. One unique approach has been to use a silica precursor, silicic acid and a small amount of a silane coupling agent to stabilize zirconia grown from zirconium oxychloride salt for polymethylmethacrylate systems [Wang, H. Xu, P., Zhong, W., Shen, L., and Du, Q.,“Transparent poly(methyl methacrylate)/silica/zirconia nanocomposites with excellent thermal stabilities”, Polymer Degradation and Stability, 87, 319-327 (2005)]. An additional salt precursor approach for making stable zirconia sols has involved the use of polyether acids which are often carboxylic acid based [Kolb, B. U. and Chien, B. T., “Zirconia Sol, Process of Making and Composite Material”, U.S. Pat. No. 6,376,590 B2 (2002)].
In the case of zirconium alkoxides, they react vigorously with water, and if the rate of the condensation reaction is not controlled, an immediate precipitation of hydrated zirconia occurs. Using a zirconia alkoxide precursor and a surfactant by itself does not prevent agglomeration of the zirconia in polymethylmethacrylate systems, even when using water-in-oil microemulsions due to the reactivity of the zirconia precursor [Palkovits, R., Althues, H. Rumplecker, A., Tesche, B. Dreier, A., Holle, U., Fink, G., Cheng, C. H. Shantz, D. F. and Kaskel, S., “Polymerization of w/o Microemulsions for the Preparation of Transparent SiO2/PMMA Nanocomposites”, Langmuir, 21, 6048-6053 (2005)]. Some success in PMMA systems has been accomplished by initial transesterfication of zirconia alkoxides with 2-hydroxyethyl methacrylate and then free radical polymerization with methylmethacrylate monomer [Di Maggio, R., Fambri, L., and Guerriero. A., Chem. Mater., 10, 1777 (1998]. In the present invention, the reactivity of zirconium alkoxide with water is retarded by reacting them with PDMS molecules with terminal OH groups to form zirconia/PDMS nanosized clusters.
The configuration or the structure of zirconia/PDMS nanosized clusters can be modified by varying the ratio of the OH groups coming from the PDMS oligomer ([OH]PDMS) to Zr. For instance, theoretically, a [OH]PDMS/Zr ratio of 1 will lead to a linear zirconia/PDMS cluster (referred as Structure I, shown in FIG. 1), while raising the ratio to 2 will result in a tetrameric zirconia/PDMS cluster (referred as Structure II, shown in FIG. 2). It is anticipated that the functional groups (e.g., OH, —CH═CH2) on zirconia/PDMS clusters will react with ≡Si—H groups of the component B of the silicone coating formulation to form chemical bonding between the elastomeric silicone network and zirconia/PDMS nanosized clusters. Mechanical properties of the silicone coating will depend upon the structure and size of zirconia/PDMS clusters and the extent of cross-linking between clusters and elastomeric silicone network because chemically bonded zirconia/PDMS nanosized clusters can assist in forming a bimodal silicone elastomeric network, i.e. a network composed of both short and long chains.
In bimodal networks, short chains are end-linked with relatively long chains. It is postulated that while short chains contribute in enhancing the ultimate strength of elastomers, the long chains control the elongation-to-break ratio of the elastomer [J. E. Mark, Acc. Chem. Res., 37, 946-953 (2004)]. On the other hand, organically modified platelet-shaped clay particles act more as nanoscale fillers and do not alter the elastomeric network, but allow load transfer to the matrix at relatively small weight fraction because of their nanosized nature.